The present invention relates generally to electromechanical linear actuators. More particularly, it relates to a lead screw-driven linear actuator having a specially constructed guide chassis that encloses the lead screw and the drive nut and protects them from dirt, debris and contamination.
Linear actuators are used in a great many machine assemblies to provide linear motion between two parts of the machine, for example to provide linear motion between a machine base and a load. Different types of linear actuators include pneumatic cylinders, hydraulic cylinders, rodless cylinders, rack-and-pinion gears, lead screw actuators and ball screw actuators. Lead screw actuators generally have a rotatable lead screw and a drive nut that engages the helical threads of the lead screw. When the lead screw is rotated relative to the drive nut, the drive nut converts the rotary motion to linear motion. In some applications, the rotatable lead screw is held stationary and the drive nut, which is constrained from rotating, moves linearly relative to the lead screw. In other applications, the drive nut is held stationary and the rotatable lead screw moves linearly relative to the drive nut. In yet another variation, the lead screw is constrained from rotating and the drive nut is rotated to create linear motion of the lead screw.
Ball screw actuators are a special type of lead screw actuator in which the drive nut contains recirculating ball bearings that engage the helical threads of the lead screw. The recirculating ball bearings reduce the friction between the lead screw and the drive nut, providing a highly efficient conversion of rotary motion to linear motion. Lead screw actuators have many advantages including easily controlled speeds, reversibility, precise and repeatable positioning of loads, and high resistance to being backdriven by forces on the load.
Contamination resistance is important in many applications of linear actuators. For example, in a clean room environment, it is important that the mechanism of a linear actuator does not release debris that would contaminate the operating environment. Conversely, in highly contaminated environments, the mechanism of the linear actuator must be protected from environmental debris and contamination. In nearly all environments, it is desirable to protect the mechanism of the linear actuator from mechanical damage as well.
One prior art attempt to create a contamination resistant lead screw actuator is described in U.S. Pat. No. 5,915,916. This patent and all other U.S. patents referred to herein are hereby incorporated by reference in their entirety. The lead screw and drive nut are enclosed within a housing that has a slit in it. A moving carriage is attached to the drive nut through the slit. A moving seal belt, which is mounted on pulleys, attaches to the carriage and covers the portion of the slit that is not occupied by the carriage. In other prior art devices, the lead screw and drive nut or other mechanism of a linear actuator is enclosed in a U-shaped channel and telescoping or accordion-folded panels cover the open top of the channel to exclude debris and contamination. These prior art devices tend to be overly complex, expensive and subject to mechanical failure.
Current manufacturing processes for linear actuators in general, and lead screw actuators in particular, involve the use of very expensive precision manufacturing equipment and very often involve the machining and grinding of hardened steels, which both add significantly to the manufacturing costs. One main source of the expense is the production of the linear rails that are part of the guide assembly in a linear actuator. U.S. Pat. No. 6,052,902 represents one prior art attempt to reduce the complexity and expense of linear motion bearing fabrication. The fabrication method described does not go far enough in eliminating the expensive precision manufacturing processes involved in fabricating a linear actuator assembly. Thus, there is a continuing need for improvements to the current manufacturing processes for fabricating a guide assembly for use in linear actuators.
It would be desirable, therefore, to provide a contamination resistant linear actuator, particularly a lead screw actuator, that is simple, low cost to manufacture and mechanically reliable.
In keeping with the foregoing discussion, the present invention provides a contamination resistant lead screw-driven linear actuator in which the lead screw and drive nut are enclosed within a specially constructed guide chassis that protects them from dirt, debris and contamination. The guide chassis also serves to effectively contain any debris or contamination produced by the linear actuator mechanism so that it does not contaminate the operating environment of the linear actuator.
The linear actuator is constructed around a guide assembly that includes a guide chassis to which are attached a pair of parallel guide rails. A carriage is slidably mounted on the guide chassis by bearing assemblies that are supported on the parallel guide rails. One or more magnets are attached to the carriage to facilitate non-contact position sensing. A drive nut, which preferably includes an anti-backlash mechanism, is driven back and forth by a lead screw, which is in turn driven by a reversible electric motor. The lead screw and the drive nut are enclosed within an approximately cylindrical central passage within the guide chassis. A narrow slot through the wall of the guide chassis extends along the length of the central passage. The drive nut is attached to the carriage through the slot in the guide chassis by a nut flange. The slot in the guide chassis is preferably angled downward away from the top of the guide assembly where the carriage is mounted. This configuration helps to resist environmental debris from entering the central passage and contaminating the lead screw and drive nut.
The linear actuator has an enclosure that includes a side cover plates and an end cover plate, which cover the sides and end of the guide subassembly, and a molded plastic motor housing, which covers the electric motor. Hall effect position sensors are repositionably mounted on the exterior of the enclosure to sense the position of the carriage. The enclosure, together with the guide chassis, creates a labyrinth seal that effectively contains internally generated debris and contamination and excludes environmental dirt, debris and contamination.
In a preferred embodiment, the linear actuator is provided as a fully assembled integrated module ready to install for a variety of linear motion applications. In a typical application, the guide chassis of the linear actuator is affixed to a base, such as a work surface or a machine, and a load, which may be a mechanical or electromechanical device, a tool, a fixture, an optical component, etc., is mounted on the carriage of the linear actuator. The carriage is driven back and forth along the length of the parallel guide rails by the electric motor to position the load with respect to the linear actuator.
The present invention also provides an improved method of manufacturing a screw-driven linear actuator.